The present invention relates generally to thermal energy recovery systems. More particularly, the present invention relates to the efficient, controlled operation of an Organic Rankine Cycle (ORC) system in which waste heat is recovered from a reciprocating engine and/or a natural gas compressor coupled to a reciprocating engine.
Methods for implementing a Rankine cycle within a system to recover thermal energy from a heat source are well known. Although most waste heat recovery systems were initially developed to produce steam that could be used to drive a steam turbine, the basic principles of the Rankine cycle have since been extended to lower temperature applications by the use of organic propellants within the system. Such ORC systems are typically used within thermal energy recovery systems or geothermal applications, in which heat is converted into mechanical energy that can be used to generate electrical energy. As such, these systems have become particularly useful in heat recovery and power generation—collecting heat from turbine engine exhaust, combustion processes, geothermal sources, solar thermal energy collectors, and thermal energy from other industrial sources.
Generally, a Rankine-based heat recovery system includes a propellant pump for circulating propellant throughout the system, an evaporator for evaporating propellant that has become heated by collection of waste heat, an expander (typically a turbo-expander) through which evaporated propellant is allowed to expand and create power or perform work, and a condenser for cooling the propellant back to liquid state so it may be pumped to again collect heat and repeat the cycle. The basic Rankine cycle has been adapted for collection of heat from various sources, with conversion of the heat energy to other energy outputs.
For example, U.S. Pat. No. 5,440,882 describes a method for using geothermal energy to drive a modified ORC based system that uses an ammonia and water mixture as the propellant. The evaporated working fluid is used to operate a second turbine, generating additional power. Heat is conserved within the Rankine cycle portion of the system through the use of a recuperator heat exchanger at the working fluid condensation stage.
U.S. Pat. No. 6,986,251 describes an ORC system for extracting waste heat from several sources in a reciprocating engine system. A primary propellant pump drives the Rankine cyde with assistance from the auxiliary booster pump, to limit pump speeds and avoid cavitation. When the Rankine cycle is inactive (e.g. due to reciprocating engine failure or maintenance), the auxiliary pump continues to operate alone, circulating propellant until the propellant and system components have cooled sufficiently for complete shut down. Diversions are present to prevent circulation of propellant through the evaporator and through the turbine during this cooling cycle.
U.S. Pat. No. 4,228,657 describes the use of a screw expander within a Rankine cycle system. The screw expander is used to expand a propellant, and waste heat is further extracted from the expander in order to improve system efficiency. A geothermal well supplies pressurized hot water or brine as the heat source.
When using organic propellants within a Rankine cycle, care must be taken to avoid exposure of the propellants to flame. Although specialized organic propellants having high flash temperatures (for example Genetron® R-245fa, which is 1,1,1,3,3-pentafluoropropane) have been developed, the danger of combustibility still exists, as engine exhaust may reach temperatures up to 1200 degrees F. Further, the purchase of proprietary propellants adds a significant cost to these systems and requires the ORC system to be in close proximity to the heat source.
A common problem particularly relevant to recovery of thermal energy is that when using air-cooled condensers, ambient air temperatures significantly impact the ORC system efficiency and total power generated. Applicant's co-pending application, WO 2008/106774, describes a robust configuration and associated operation of an ORC system, with heat collection from various waste heat sources driving evaporation of propellant to provide secondary energy output. This secondary power may be used to directly power parasitic loads within the system, enabling independent control and operation of these loads, improving system efficiency. One notable application of this system lies in the compression of natural gas at both on-grid and off-grid sites for pipeline transport, with the reciprocating engine driving the natural gas compressor.
Published application WO 2006/138459 describes an ORC system in which an organic propellant is used to remove heat from the engine.
Retrofitted systems for recovering heat from a reciprocating engine are generally limited by pre-existing space constraints and site conditions, particularly when used in remote locations. Generally, heat recovery systems of the prior art require close proximity to the engine, liquid condensing, expensive components, do not incorporate a recuperator (economizer) and are not sufficiently adaptable to recover and utilize thermal energy from other sources, if present.